Discussion
High variation of sizes, shapes, and p/ D ratios were observed in California Quercus species. The differences of sizes between evergreen and deciduous oak pollen were found in California species. The shorter furrows in deciduous oak pollen than that in evergreen oak pollen was observed. Quercus douglasii , Quercus garryana , and Quercus kelloggii display higher p/D ratios than other species. These 3 species are coincided with 3/4 deciduous oaks (not including Quercus Xmacdonaldii ) in samples. The relationship between this feature and deciduous leaves or dry environment adaptation is not clear.
The variation of sizes, shapes, and surface ornamentation of pollen inQuercus section is the highest and average pollen size is the largest in three sections. That might be resulted from the larger number of species categorized in this section. New World Quercussection, Protobalanus , has the smallest average size and variation in three studied sections. This group is unique by its smooth pollen surface, dense scabrate/rodlike masked (Denk and Grimm, 2009). The higher similarity of sizes between Quercus and Lobataesections coincide with the closer phylogenetic distance of these two sections than them and Protobalanus (Figure 1 and 2; Mano et al., 2001; Denk and Grimm, 2009). We applied T-test on 20 species (excluding hybrid QuercusChrysolepis , and Notholithocarpus ). The results suggest that there are significant different between sizes and shapes of each species, each section, and evergreen-decidous oaks. I applied Moran’s tests on 20 species to test phylogenetic signals. There is only the length of polar view passed Moran’s test (p <0.05; p = 0.041 and 0.042 in 2 tests) when I tested 20 species at the same time. It suggested that the length of polar view of the oak pollen grains contain phylogenetic signals. I also found phylogenetic signal in the distance between 2 furrows in Lobatae sections, Q. agrifolia, Q. kelloggii, Q. parvula, and Q. wislizeni (p <0.05; p =0.035). Further investigations will be focused on details of surface ultrastructure and trait-environment relationships.
Scanning electron microscopy (SEM) images provide details of surface patterns of Quercus pollen. Former SEM studies about oak pollen in the United States only display comparison of Fagaceae phylogenies and several images of pollen structure, such as Solomon’s (1983) pollen morphologic studies about white and red oaks in eastern North America and Médus’ and Flores’ (1984) pollen morphology of some Mexican oaks. Based on the terminology and classification from Denk’s and Grimm’s (2009), oak pollen grains in Quercus and Lobatae sections in this study can be classified into “(micro) verrucate” type. Quercus cedrosensis , Quercus chrysolepis , Quercus palmeri , Quercus tomentella , Quercus parvula , and Quercus wislizeni from Protobalanus were classified into “rodlike masked” type (Figure 10).
Because of the gradually changes in rodlike masked sculpture, Quercus pollen surface ornamentation of studied species were be able to classified into another 3 types. Type 1 includes Q. agrifolia, Q. cornelius-mulleri, Q. dumosa, Q. durata, Q. engelmannii, and Q. Xmacdonaldii. Type 2 includes Q. kelloggii, Q. turbinella, Q. pacifica, Q. garryana, Q. john-tuckeri, Q. lobate, Q. wislizeni, Q. palmeri, Q. parvula, Q. saddleriana, Q. douglasii, and Q. Xalvordiana. Type 3 includes Q. cedrosensis, Q. vaccinifolia, Q. chrysolepis, and Q. tomentella. The surface elements on Type 1 Quercus pollen grains are blocks (verrucate) with sparse micro-elements on them. Type 2 pollen grains posse big blocks with dense micro-elements on them. Type 1 and type 2 are both included to “(micro) verrucate” category in Denk’s and Grimm’s classification. Type 3 can be matched to rodlike masked category (Figure 10).
The pollen morphologies of Chrysolepis chrysophylla and Notholithocarpus densiflorus var echinoides , support their closer phylogenetic relationships than them to Quercus genus. High endemism of their geographical distribution provides evidence for the relictual nature of the castaneoids occurring in western North America. The differential pollen morphologies coincide with their insect pollination mechanism which is different to wind pollination of Quercus .